Oye! Well, the only way she could actually be pregnant is if she was with Brian within a month after his surgery (still viable wrigglies)
or she was somehow exposed to another intact male within the last month. It is not uncommon for an intact female to go through a false pregnancy if she’s mated with a neutered male. So if she is dealing with a false pregnancy then, on average, it can last for about three weeks.
Unfortunately there’s not a whole lot you can do except wait it out. Is she acting more aggressive at all towards you and/or Brian. She may get more territorial as her instincts are telling her to protect her "prized" nesting location. So you may have to watch her interaction with Brian more closely to prevent a fight from breaking out, and causing them to unbond.
EDITED TO ADD: Oh, I forgot to mention – if she could be possibly pregnant then you will want to make or get a nesting box. Just a box where the babies can stay safe and warm. If you think she really could be, let me know, and I’ll continue to give advice for actual pregnancy.